The present invention generally relates to brake systems, and more particularly relates to an improved vacuum brake force booster for use in a hydraulic brake system.
Vacuum brake force boosters in many applications are known in the automotive industry. The brake force is typically transmitted to the vacuum brake force booster by way of the pedal push rod which is in connection to a brake pedal, and ultimately to the master cylinder connected to the booster. The same vacuum prevails on both sides of the diaphragm plate in the release or ready position. An associated compression spring within the vacuum housing or pneumatic cylinder pressure housing retains the diaphragm plate in its initial position. An associated vacuum channel is open which pneumatically separates the two cylinder chambers from each other by way of the poppet valve.
Upon application of the brake pedal, the above-mentioned vacuum channel is initially closed by the valve piston with the connected poppet valve, and an outside air channel is opened in the further course of the movement so that the vacuum which prevails in the working pressure chamber is decreased. Consequently, there is a difference in pressure between the front and the rear chamber of the booster (pedal-side working pressure chamber or booster chamber) which pushes the diaphragm plate in the direction of the connected (tandem) master cylinder and thus generates the pedal force. The brake boosting or auxiliary force which is so produced is rated by the product of differential pressure and diaphragm plate surface. The above-mentioned vacuum reduction in the booster chamber is effected until the hydraulic pressure generated in the connected master cylinder makes the valve piston come to standstill. Simultaneously, the poppet valve interrupts the supply of outside air. Now a ready position is reached where each slight variation of the pressure at the brake pedal causes an increase or decrease of the pressure difference on both sides of the diaphragm plate and, thus, an increase or reduction in slowing down.
In the so-called brake""s fully applied position, the vacuum channel is closed and the outside air channel is constantly open. The result is that the maximum pressure difference and, hence, maximum brake force boosting prevails on both sides of the diaphragm plate. Further increase of the brake force is possible only by increase of the pedal force.
In a vacuum brake force booster of the design initially described, there is provision of an air filter which is principally arranged in a sleeve that encompasses the pedal push rod. In addition, it is suggested to apply sound-absorbing material configured as a hollow-cylinder shaped lining to the inside wall of the sleeve, and the above-mentioned sound-absorbing material encompasses the pedal push rod at a radial distance therefrom so that the air which flows through an annular channel flows along the sound-absorbing lining. Attempts have been made with this arrangement to considerably absorb the noise of air aspiration in a vacuum brake force booster without increasing the diameter in the area of the air-conveying channels and without extending the response time of the brake force booster (see German patent specification 29 18 734).
The above-mentioned embodiment still suffers from disadvantages, especially with respect to the expenditure in manufacture. Thus, it is necessary in this embodiment to cement the hollow-cylindrical lining of sound-absorbing material to the sleeve. It is scarcely possible to automate such processes and, on top of this, manufacturing inaccuracies are difficult to master. The present invention is directed to this issue.
A technical object of the present invention is to improve upon a vacuum brake force booster of the design described hereinabove so that a simple and low-cost manufacture is achieved along with proper filtering and/or sound absorption of the outside air.
This object is achieved by the present invention in a generic brake force booster in that the air filter is arranged in or at the valve piston and, in conjunction with an associated valve piston sealing seat (for the air filter abuttable thereon), forms a variable flow cross-section for the outside air that flows through the poppet valve. The outside air flows through the air filter and/or past it. With respect to its outside dimensions, the air filter can project from the valve piston sealing seat, end flush with the sealing seat of the valve piston, or be disposed flatly in the valve piston.
In a preferred embodiment, the air filter is an annular disc with a central bore for the pedal push rod extending therethrough. Consequently, a simple wholesale article of low-cost manufacture may be used. This applies especially for the case that the air filter is made from an elastic material of a predetermined flexibility, for example, a PUR (polyurethane) foam, or is designed rigidly. In addition or as an alternative, there is the possibility of connecting the air filter to the valve piston by means of a spring or any other elastically yielding or flexible element.
All these provisions are generally made because the valve piston sealing seat typically has at its rim a baffle plate with a protruding stop edge. This stop edge may project by a predetermined extent from a valve seat plane defined by the valve seat. Of course, a receding type of construction is also possible. The stop edge will regularly plunge into the air filter, when the air filter bears against the valve seat or has lifted from the valve seat by a predetermined slot size, so that outside air which flows through the poppet valve is forced to pass the air filter. Consequently, the outside air is initially conducted through an annular channel between the pedal push rod and the sleeve which receives the pedal push rod, and then in an S shape through the air filter and into the working pressure chamber. This arrangement achieves a proper sound absorption in the above-mentioned position of the valve piston or the poppet valve because the outside air which flows into the working pressure chamber must obligatorily pass the air filter (which is usually made of a foam material). Such a position generally corresponds to the ready position or partial braking position.
In full braking or a quick application of the brake pedal and, thus, of the pedal push rod (emergency braking), the air filter is removed from the valve seat by a predetermined opening rate. Simultaneously, the stop edge no longer plunges into the air filter, rather is lifted from the air filter. Thus, the stop edge along with the air filter defines an intake so that outside air which flows through the poppet valve is deviated by the air filter (S-shape) and flows in through the intake. In this case, too, proper sound absorption will be achieved and namely in such a way that the outside air which enters through the annular channel so-to-speak impinges on the air filter and is deviated by said into the intake. Thus, a sound absorption is done in two different ways: on the one hand, by the porous and sound-absorbing surface of the filter being an impingement surface and, on the other hand, by the deviation of the air flow.
An embodiment which is especially favorable under aspects of manufacture and assembly is characterized in that the valve piston includes on the side of the pedal push rod an extension of U-shaped cross-section, formed with a cavity for insertion of the air filter. This extension and the air filter can be designed rotationally symmetrically in comparison to the axis defined by the pedal rod. In general, the air filter and the extension are arranged coaxially and concentrically with respect to the centrally arranged pedal push rod.
The result is a structural unit which is especially simple to manufacture and easy to mount.
First of all, this is due to the air filter which can be configured as a punched plastic (foam) shaped part in the most simple case. The assembly is also easy because the valve piston includes a cavity for insertion which is conformed to the outside diameter of the air filter in a practical fashion and permits positioning the air filter without problems. An additional adhesive securement of the air filter in the insertion cavity is possible, but not absolutely necessary. Compared to the state of the art as disclosed in DE-PS 29 18 734, special designs on the sleeve which encompasses the pedal push rod become unnecessary, with the result of reduced manufacturing and assembly costs. At least the same noise-absorbing properties as in the state of the art, or even better ones, will be achieved. The air filter inheres a defined flexibility (elasticity) so that the air is either passed through it or past it. The latter action is required in particular upon a quick actuation of the valve piston, namely in connection with a quasi emergency braking. The above-mentioned mode of operation causes a particularly quick ventilation of the working pressure chamber or of the booster chamber because the entry of outside air takes place practically unimpededly. In the first-mentioned case, the flexibility of the filter element is sufficient to effect the conveying of the outside air through the filter by means of the baffle plate. The available flow cross-section is sufficient for normal service brake operations. These are the basic advantages of the present invention.